将枚举的最后一个元素返回为Rust中的Default

Question

我有一个生锈实现如下，在默认实现中，对于枚举，我需要返回最后一个元素，我需要实现它而不需要对其进行硬编码。

#[cfg_attr(feature = "std", derive(Debug))]
#[derive(Clone,  PartialEq, Eq)]
pub enum Region<CountryId> {
    None,
    Category(CountryId),
}

#[cfg_attr(feature = "std", derive(Debug))]
#[derive(Clone, PartialEq, Eq)]
pub struct Litrature1<CountryId> {
    pub Seek: Region<CountryId>,
    pub Write: Region<CountryId>,
}

#[cfg_attr(feature = "std", derive(Debug))]
#[derive(Clone, PartialEq, Eq)]
pub struct Litrature2<CountryId> {
    pub Seek: Region<CountryId>,
    pub Write: Region<CountryId>,
    pub Work: Region<CountryId>,
}

#[cfg_attr(feature = "std", derive(Debug))]
#[derive(Clone,  PartialEq, Eq)]
pub enum Alphabets<CountryId> {
    A1(Litrature1<CountryId>),
    A2(Litrature1<CountryId>)
}


impl<CountryId> Default for Alphabets<CountryId> {
    fn default() -> Self {
        // How to return the last element of the enum as default?
        Alphabets<CountryId>::A2
    }
}

Playground

我不确定如何使这项工作

Answer 1

我假设您基本上想要默认值，每个区域默认为Region::None。在这种情况下，最有意义的是在每个嵌套类型上定义Default，例如

Region的默认值

impl<CountryId> Default for Region<CountryId> {
    fn default() -> Self {
        Region::None
    }
}

Litrature1的默认值

impl<CountryId> Default for Litrature1<CountryId> {
    fn default() -> Self {
        Litrature1 {
            Seek: Default::default(),
            Write: Default::default(),
        }
    }
}

Litrature2的默认值

impl<CountryId> Default for Litrature2<CountryId> {
    fn default() -> Self {
        Litrature2 {
            Seek: Default::default(),
            Write: Default::default(),
            Work: Default::default(),
        }
    }
}

Alphabets的默认值

impl<CountryId> Default for Alphabets<CountryId> {
    fn default() -> Self {
        Alphabets::A2(Default::default())
    }
}

（Qazxswpoi）